The invention relates to an integrated electromechanical microstructure comprising a base substrate and a cavity closed by a protective cover, and a process for adjusting the pressure in the cavity.
Integrated electromechanical microstructures or MEMS (Micro Electro-Mechanical Systems) using manufacturing processes stemming from micro-electronics are increasingly used, in particular for manufacturing accelerometers, gyrometers for navigation and RF or optic switches for telecommunications.
As represented schematically in FIG. 1, such a microstructure conventionally comprises a base substrate 1. The mobile mechanical elements 2 of the microstructure are arranged in a microcavity 3. The latter is closed by a protective cover 4 by means of a peripheral sealing seam 5. To reduce costs, several microstructures are generally manufactured simultaneously and a protective cover common to all the microstructures is sealed before the different microstructures are cut off.
The performances of the mechanical elements of the microstructures, such as the thermal noise, damping or passband, depend directly on the gaseous environment surrounding these elements inside the cavity 3. In current manufacturing processes, the pressure inside the cavity 3 is fixed by the surrounding pressure at the time sealing is performed. The final atmosphere inside the cavity is in fact related to the sealing process and, when several microstructures are manufactured simultaneously, it is necessarily identical in all the cavities. Moreover, it is set once and for all, depends on the sealing temperature due to the law of perfect gases, and can be polluted by degassing of the materials when sealing is performed.
It may be desired to control the atmosphere inside the cavity 3 fairly closely. It may for example be required to create a vacuum in the cavity to have a low noise or very sharp resonance peaks, or to have a pressure of a few bars for high damping and very low cut-off frequencies.
In certain cases, a sealing process in two stages is used: after the cover 4, which is initially provided with a hole, has been sealed, the pressure is set to the required value and the hole is filled. This relatively complex process does not however give total satisfaction.
The object of the invention is to overcome these shortcomings and more particularly to make control of the pressure in the cavity of an integrated electromechanical microstructure easier.
According to the invention, this object is achieved by the fact that the microstructure comprises pressure adjusting means comprising at least one element made of pyrotechnic material, combustion whereof releases gas into the cavity so as to adjust the pressure in the cavity after the protective cover has been sealed.
The pressure in the cavity can thereby be defined independently from the sealing process and the pressure in each already sealed component can be adjusted individually.
The element made of pyrotechnic material can be arranged in the cavity or in an additional cavity formed in the protective cover, a micro-orifice of the protective cover joining the two cavities.
An object of the invention is also to provide a process for adjusting the pressure in the cavity of a microstructure comprising ignition of at least one element made of pyrotechnic material after the protective cover closing the cavity has been sealed.